Vibrio cholerae (V. cholerae) is a non-invasive enteropathogen of the small bowel that does not penetrate the mucosal surface. Local Siga mediated immunity at the mucosal surface is therefore implicated as a protective mechanism. Pathogenic V. cholerae 01 elaborate a protein enterotoxin (also know as cholera enterotoxin, or choleragen, or cholera toxin) which is responsible for induction of copious secretion by the intestine resulting in watery diarrhea, the clinical consequence of cholera infection. The genes responsible for cholera enterotoxin are the ctx genes (also known as the tox genes). Cholera diarrhea can be extraordinarily severe and result in loss of so much body water and salts that dehydration, acidosis, shock, and death ensue without prompt therapy. There is known in the region of the V. cholerae chromosome containing the ctx genes that multiples copies of a 2700 base pair sequence called RS1 (for repetitive sequence) can be found. Mekalanos, Cell 35, 253-263 (1983).
Applicants have also discovered that a second enterotoxin is produced by V. cholerae which has been named zonula occludens toxin, reported in Fasano et al, Vibrio cholerae Produces a Second enterotoxin Which Affects Intestinal Tight Junctions, Proc. Nat. Acad. Sci. (USA) 88, 5242-5246 (1991).
The cholera vaccines that have been developed can be broadly divided into two categories; those aiming to stimulate antitoxic immunity and those intending to induce antibacterial immunity. Experiments with animal models support a protective role for either or both antitoxic and antibacterial immunity. It has been suggested that when both types of immunity work in unison, there is a synergistic effect. Holmgren, J. et al. J. Infect. Dis. 136 Suppl., S105-S1122 (1977); Peterson, J. W. Infect. Immun. 26, 594 (1979); Resnick, I. G. et al. Infect. Immun. 13, 375 (1980); Svennerholm, A.-M. et al. Infect. Immun. 13, 735 (1976)!. However, it appears that protective immunity in humans can be conferred without such synergistic effect, that is by either antitoxic immunity or antibacterial immunity Eubanks, E. R. et al. Infect. Immun. 15, 533 (1977); Fujita, K. et al. J. Infect. Dis. 125, 647 (1972); Holmgren, J., J. Infect. Dis., supra; Lange, S. et al. Acta Path. Microbiol. Scand Sect. C 86, 145 (1978); Peterson, J. W., supra (1979); Pierce, N. F. et al. Infect Immun. 37, 687 (1982); Pierce, N. F. et al. Infect. Immun. 21, 185 (1978); Pierce, N. F. et al. J. Infect. Dis. 135, 888 (1977); Resnick, I. G. et al., supra; Svennerholm, A.-M. et al, supra!.